Take it easy with clamping pressure

Q: As a newcomer to woodworking, I'm not sure how much clamping pressure to use on joints made with woodworker's yellow glue. Can you give me some guidelines?

—John Burns, St. Paul, Minn.

A: John, the short answer is that a good-fitting joint with the right amount of glue doesn't require tremendous pressure. The clamps just serve to hold the surfaces in contact while the glue dries.

However, let's assume that most joints fall short of perfect, and benefit from enough force to push them into complete contact. Dale Zimmerman of Franklin International, maker of Titebond woodworking glues, recommends 100 to 150 pounds per square inch (psi) for clamping softwoods and 175–250 psi for hardwoods. When we tested one-handed bar clamps (issue 139), we found that they provided pressure just into the softwood range or a bit less. Squeeze those clamps as hard as you can. But R. Bruce Hoadley, author of the book Understanding Wood, reports that other kinds of clamps, including the bottom three pictured at right, can produce far more pressure than needed. So don't go beyond "snug" when tightening those clamps.

The maximum recommended clamping pressure for most joints is 250 psi. Putting all your muscle into many common clamp styles generates excess pressure that could force out most of the glue and produce a weak bond.

For more in-depth information on gluing and clamping, visit our Gluing and Clamping section in the WOOD Store.

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(part 2 of comment) Now, the force actually applied by each clam is missing from the article. Assuming the author erroneously rated the clamps in psi instead of pounds, and thus that a pipe clamp can exert 1000lb, you need about 9 0r 10 such clamps in order to achieve the required 9600lb (of course pipe clamps will be distributed evenly). If your countertop is 2", the glueing surface is double that, and thus you need close to 20 pipe clamps to achieve the same pressure (psi).

I agree with Jim, and unfortunately this article is at best misleading (sorry). Clamps should be rate in pounds, NOT in psi. The clamping force for a given joint can only be computed once you know the clamping area and the required clamping pressure. For instance if you want to laminate a 4 ft, 1"-thick countertop, your glueing surface is 4x12x1=48 sq-in. In order to achieve the required pressure, say 200 psi, the total force should be 200x48=9600lb. (continued...)

I think the point to the article was: don't use more clamping force than needed to get you pieces flush with each other, one thing the article didn't note was that most of us can get on a given clamp enough to make it flex, if you find yourself doing that in order to get the pieces joined you need to find a stronger clamp and use more moderate hand preasure

Jim, your first comment is correct add this onto the sentence->(..20 psi per square inch). But I think what the authors are saying is if the pads on the clamps measure 1" square you would only apply the max force on that pad. yes it is distributed over the area you are gluing thus lowering the amount of force per sq. inch. Your last sentence is the way I do it, just enough to get a small bead of glue.

Chris: PSI is a measure of force (pounds) applied to an area (square inches). One clamp applying 200 pounds of force to one square inch gives 200 psi; one clamp applying 200 pounds of force to 10 square inches gives 20 psi. The article confuses force (what the clamps provide) with pressure (which depends on the size of the area being glued). Spread your glue thinly and evenly and use enough force to get an even row of droplets squeezed out.

Jim i think the answer to your question would be that, pounds per square inch relates to a measurement of pressure, like 1 inch or 32/32nds is a measurement of leigth.so if i apply 250 psi it also means that its 250 pound per inch regardless of how many inches you are appling it to, no differant than putting 32 P.S.I. of air in a tire regardless of how big the tire is.
Chris

Thanks for the answer to this one, Im just starting to get into the wood and recently purchased serveral types of clamps so i could start glueing boards. Right on time for me.
Thanks again,
Chris from St. Louis, mo.